Solid electrolytic capacitor and method of manufacturing the same

ABSTRACT

A solid electrolytic capacitor excellent in adhesion to a solid electrolyte with excellent ESR and heat resistance can be provided without reducing the material characteristics of a separator. This solid electrolytic capacitor comprises a capacitor element formed by winding an anodic foil prepared from a chemical conversion foil obtained by anodizing a metal having a valve action and a counter cathodic foil through a separator and a solid electrolyte employed as an electrolyte, while the separator is prepared from aramid fiber, and a silane coupling agent adheres to voids of the aramid fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/038,561, filed on Feb. 27, 2008 which is based upon and claims thebenefit of priority from the prior Japanese Patent Application No.2007-047738, filed on Feb. 27, 2007, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid electrolytic capacitor and amethod of manufacturing the same, and more particularly, it relates to awound type solid electrolytic capacitor and a method of manufacturingthe same.

2. Description of the Background Art

With the recent digitization and increase in frequency of electronicequipment and the increase in the temperature for reflow soldering withlead-free solder, a miniature high-volume capacitor excellent in lowimpedance characteristics in a high-frequency domain and heat resistanceis required.

Such requirements for miniaturization, increase in volume and lowimpedance characteristics in a high-frequency domain can be satisfied bya wound type electrolytic capacitor obtained by storing a capacitorelement formed by winding a cathodic foil and an anodic foil through aseparator in a metal case and sealing the same with sealing rubber. Asolid electrolytic capacitor having a solid electrolytic layer made of aconductive polymer such as polypyrrole or polythiophene exhibitingexcellent conductivity is provided.

Such a solid electrolytic capacitor is subjected to reflow solderingwith lead-free solder, having a melting point considerably higher thanthat of conventional lead solder, at a high temperature of 200 to 270°C. If the solid electrolytic capacitor having the electrolyte of theconductive polymer such as polypyrrole or polythiophene is subjected toreflow soldering under such a temperature condition, swelling of thesealing rubber or the metal case or deterioration of various electriccharacteristics disadvantageously excessively progresses due to crackedgas resulting from deterioration of the conductive polymer.

It has been recognized that the cracked gas results from deteriorationof the conductive polymer for the following reason: Thermaldecomposition of synthetic cellulose made from natural fiber generallyused for the separator starts from a temperature of about 150° C., anddeterioration of the conductive polymer acceleratedly progresses due tothis decomposition, to generate the cracked gas.

Therefore, employment of aramid fiber which is an organic polymer havinga high coefficient of elasticity and excellent heat resistance as aseparator is proposed (refer to Japanese Patent Laying-Open Nos.2002-203750 and 2002-252147, for example).

However, a solid electrolytic capacitor having a separator of aramidfiber is inferior in adhesion between a conductive polymer employed as asolid electrolyte and the separator. Consequently, equivalent seriesresistance (hereinafter abbreviated as ESR) is disadvantageouslyincreased.

The aramid fiber has high strength, a high coefficient of elasticity andhigh heat resistance due to properties such as high orientation.However, the surface of the aramid fiber is so inactive that theinterface between the aramid fiber and a polyelectrolyte is inferior instrength or adhesion. According to a method of activating the inactivesurface of the aramid fiber (refer to Japanese Patent Laying-Open No.2004-164974, for example), the heat resistance of the aramid fiber isreduced due to chemical treatment. Consequently, the materialcharacteristics of the aramid fiber for serving as the separator of asolid electrolytic capacitor are remarkably reduced.

SUMMARY OF THE INVENTION

In consideration of the aforementioned problems, an object of thepresent invention is to provide a solid electrolytic capacitor excellentin adhesion on the interface between a separator and a solidelectrolyte, having low ESR and excellent heat resistance.

In order to solve the aforementioned problems, the present inventionprovides a solid electrolytic capacitor comprising a capacitor elementformed by winding an anodic foil provided with a dielectric film and acounter cathodic foil through a separator and impregnated with a solidelectrolyte, characterized in that a silane coupling agent adheres tofiber constituting the separator. This separator is preferablyconstituted of aramid fiber.

The present invention also provides a method of manufacturing a solidelectrolytic capacitor, comprising the step of winding an anodic foilprovided with a dielectric film and a counter cathodic foil through aseparator, and further comprising the step of dipping the separator in asolution containing a silane coupling agent in advance of the step ofwinding the anodic foil and the counter cathodic foil through theseparator.

In the solution containing the silane coupling agent, a supercriticalfluid is preferably employed as a solvent.

The method further the step of performing heat treatment on theseparator in a vacuum after the step of dipping the separator in thesolution containing a silane coupling agent. The temperature for theheat treatment is preferably 80 to 200° C.

The separator according to the present invention is so employed that asolid electrolytic capacitor excellent in heat resistance and ESR can beprovided without damaging the characteristics of the separator.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a capacitor element of a solidelectrolytic capacitor according to an embodiment of the presentinvention; and

FIG. 2 is a sectional view of the solid electrolytic capacitor accordingto the embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention is now described with referenceto the drawings.

FIG. 1 is a sectional view of a capacitor element of a solidelectrolytic capacitor according to the embodiment of the presentinvention. An anodic foil 2 made of a valve action metal such asaluminum and provided with an oxide film layer on the surface thereofand a cathodic foil 3 are wound through a separator 4 and fixed with asealing tape 5, thereby forming a capacitor element 1. An anode leadwire 7 and a cathode lead wire 8 are connected to anodic foil 2 andcathodic foil 3 respectively.

Separator 4 is made of aramid fiber containing a silane coupling agentadhering to filaments. This aramid fiber is not particularly restrictedso far as the same is prepared from total paraaromatic polyamide, andmay be in the form of spun continuous fiber or single fiber cut afterspinning The thickness of separator 4 is preferably 20 to 60 μm. Theinsulation resistance of the solid electrolytic capacitor is reduced ifthe thickness of separator 4 is less than 20 μm, while the ESR of thesolid electrolytic capacitor may be increased if the thickness ofseparator 4 is in excess of 60 μm. The density of separator 4 ispreferably 0.2 to 0.7 g/cm³. The tensile strength is insufficient if thedensity of separator 4 is less than 0.2 g/cm³, while the capacitance maybe reduced and the ESR may be increased if the density of separator 4 isin excess of 0.7 g/cm³. Further, the tensile strength of separator 4 ispreferably at least 0.8 kgf/15 mm, more preferably at least 1.5 kgf/15mm. If less than 0.8 kgf/15 mm, the tensile strength may be insufficientfor separator 4 to be wound with anodic and cathodic foils 2 and 3.

The silane coupling agent can be prepared from a compound selected fromgenerally employed γ-(2-aminoethyl) aminopropyltrimethoxysilane,γ-(2-aminoethyl) aminopropylmethyldimethoxysilane,γ-methacryloxypropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane,γ-mercaptopropyltrimethoxysilane, methyl trimethoxysilane, methyltriethoxysilane, vinyl triacetoxysilane, γ-chloropropyltrimethoxysilane,γ-anilinopropyltrimethoxysilane, vinyl trimethoxysilane,γ-chloropropylmethyldimethoxysilane,γ-mercaptopropylmethyldimethoxysilane, methyl trichlorosilane,dimethyldichlorosilane, trimethylchlorosilane and the like. Inparticular, γ-(2-aminoethyl) aminopropyltrimethoxysilane is preferable.

The aramid fiber is dipped in an aqueous solution or an emulsioncontaining the silane coupling agent or a supercritical fluid so thatthe silane coupling agent adheres to voids of the aramid fiber, whilethe supercritical fluid is preferably employed in order to improve theeffect. The term “supercritical fluid” denotes a fluid in a stateexceeding the limit temperature/pressure (critical point) allowingcoexistence of a gas and a liquid, having the characteristics of both ofthe liquid and the gas. The supercritical fluid freely flows andinfiltrates into another object due to the dissolubility of the liquidand the excellent dispersibility of the gas. When the aramid fiber istreated in the supercritical fluid, the agent can largely infiltrateinto deeper portions of the voids as compared with a case where thearamid fiber is treated in a general solvent, due to the characteristicsof the supercritical fluid. The supercritical fluid is formed only undera high pressure, whereby the agent can effectively infiltrate into thevoids when the aramid fiber is treated in the supercritical fluid.Further, the supercritical fluid is vaporized under a low temperature,to hardly cause environmental problems.

While the supercritical fluid can be prepared from water, carbondioxide, alcohol or ammonia, carbon dioxide is preferably employed inconsideration of easiness in handling and the cost.

The aramid fiber constituting separator 4 is preferably dipped in thesupercritical fluid containing the silane coupling agent, dried andthereafter heat-treated in a vacuum. The crystalline size of the silanecoupling agent is increased and intercrystalline clearances are narrowedby this heat treatment, so that the silane coupling agent cansufficiently bond and adhere to the aramid fiber. The heat treatment ispreferably performed in the temperature range of 80 to 200° C. If theheat treatment is insufficient, moisture may remain in the aramid fiberto cause deterioration of a solid electrolyte upon reflow soldering ofthe solid electrolytic capacitor. Further, a cracked gas generated fromthe solid electrolyte may swell a sealing rubber packing or an armoringcase described later.

When the aforementioned treatment and the heat treatment in a vacuum areperformed, the silane coupling agent can not only infiltrate into theclearances of the aramid fiber but also seal the clearances between thefilaments of the aramid fiber as intercalations, so that the aramidfiber has a low moisture content. In the treatment with the silanecoupling agent and the heat treatment, the silane coupling agent and thearamid fiber are not chemically bonded to each other but the silanecoupling agent infiltrates into the aramid fiber through spatial changesof physical intercrystalline clearances, whereby the physical propertiesspecific to the aramid fiber can be maintained.

The aforementioned capacitor element 1 is impregnated with a solutioncontaining at least a monomer and an oxidizer and heat-treated, to beprovided with the solid electrolyte. While the monomer is notparticularly restricted so far as the same is heterocyclic and can forma conductive polymer, 3,4-ethylene dioxythiophene, pyrrole, aniline or aderivative thereof is preferable due to the excellent conductivitythereof.

Referring to FIG. 2, capacitor element 1 provided with the solidelectrolyte is stored in an aluminum armoring case 9 in the form of abottomed cylinder mounted with a sealing rubber packing 10 while leadtab terminals 6 are mounted on the bases of anode lead wire 7 andcathode lead wire 8 respectively. An opening of armoring case 9 istransversely drawn and curled for sealing capacitor element 1, which inturn is aged. Thereafter a plastic plate 11 is inserted into the curledportion, and anode and cathode lead wires 7 and 8 of capacitor element 1are pressed and folded as electrode terminals 12, for completing thesolid electrolytic capacitor according to this embodiment.

EXAMPLES

Examples of the present invention are now described.

Example 1

(Treatment of Aramid Fiber with Silane Coupling Agent)

Aramid fiber for forming a separator was dipped in a solution preparedby dissolving γ-(2-aminoethyl) aminopropyltrimethoxysilane employed as asilane coupling agent in distilled water at the ratio of 2.0 wt. %.Thereafter moisture was removed with a freeze drier, for preparing atreating agent.

The aramid fiber was dipped in a carbon dioxide fluid atmospherecontaining the treating agent prepared in the aforementioned manner, andpreserved at the room temperature for two months.

The aramid fiber was in the form of single fiber cut after spinning,with a thickness of 50 μm.

(Preparation of Solid Electrolytic Capacitor)

Referring to FIGS. 1 and 2, an anodic foil 2 was prepared from analuminum foil subjected to etching and chemical conversion, wound on acylinder along with a counter cathodic foil 3 through a separator 4 ofthe aramid fiber surface-treated in the aforementioned manner and fixedwith a sealing tape 5, thereby forming a capacitor element 1. Lead tabterminals 6, an anode lead wire 7 and a cathode lead wire 8 wereconnected to anodic foil 2 and cathodic foil 3 respectively.

Then, a cut area of capacitor element 1 was chemically converted andheat-treated at 280° C. Then, capacitor element 1 was dipped in 40 wt. %of ferric p-toluenesulfonate containing n-butyl alcohol as a diluent and3,4-ethylene dioxythophene, and a conductive polymer layer wasthereafter formed between the electrodes of capacitor element 1. Asealing rubber packing 10 was inserted into capacitor element 1, whichin turn was stored in an armoring case 9, an opening of armoring case 9was transversely drawn and curled for sealing capacitor element 1, andcapacitor element 1 was aged. A plastic plate 11 was inserted into thecurled surface of armoring case 9, and anode lead wire 7 and cathodelead wire 8 of capacitor element 1 were pressed and folded as electrodeterminals 12, for completing a solid electrolytic capacitor.

Example 2

A solid electrolytic capacitor was prepared similarly to Example 1,except that a separator 4 was heat-treated in a vacuum at a temperatureof 170° C. for 30 minutes after treatment of aramid fiber with a silanecoupling agent.

Comparative Example 1

A solid electrolytic capacitor was prepared similarly to Example 1,except that aramid fiber was not treated with a silane coupling agent.

Comparative Example 2

A solid electrolytic capacitor was prepared similarly to Example 2,except that aramid fiber was not treated with a silane coupling agentbut subjected to only heat treatment in a vacuum at a temperature of170° C. for 30 minutes.

As to the solid electrolytic capacitors prepared according to Examples 1and 2 and comparative examples 1 and 2, capacitances (initialcapacitances) at a frequency of 120 kHz, ESR values (initial ESR values)at a frequency of 100 kHz, capacitance loss factors before and afterreflow soldering, ESR changes before and after a reflow test and numbersof appearance defectives were obtained. Assuming that C₀ [μF] representsthe initial capacitance and C [μF] represents the capacitance afterreflow soldering, the capacitance loss factor ΔC [%] is obtained asfollows:

ΔC=(C−C ₀)/C ₀×100  (1)

Assuming that R₀ [mΩ] represents the initial ESR and R [mΩ] representsthe ESR after reflow soldering, the ESR change ΔR is obtained asfollows:

ΔR=R/R ₀  (2)

The reflow test was conducted at a temperature of at least 230° C. withthe maximum temperature of 250° C. for at least 30 seconds. Table 1shows the results of the reflow test.

TABLE 1 Initial Capacitance Capacitance Initial ESR Loss Factor ESRChange [μF] [mΩ] [%] [times] Example 1 151 29.3 −2.8 1.01 Example 2 15229.7 −3.0 1.07 Comparative 151 32.3 −4.9 1.43 Example 1 Comparative 15032.9 −4.8 1.23 Example 2

It is understood from Table 1 that the initial ESR values are remarkablysmall and the ESR changes before and after reflow soldering are alsosmall in the solid electrolytic capacitors according to Examples 1 and 2treated in the supercritical fluids as compared with the untreated solidelectrolytic capacitor according to comparative example 1 and the solidelectrolytic capacitor according to comparative example 2 not treated ina supercritical fluid. Comparing the solid electrolytic capacitorsaccording to Examples 1 and 2 with those according to comparativeexamples 1 and 2, it is also understood that the initial capacitances ofthese solid electrolytic capacitors are substantially equivalent to eachother while the capacitance losses before and after reflow soldering aresmaller in the solid electrolytic capacitors according to Examples 1 and2 than those according to comparative examples 1 and 2. Thus, it isunderstood that the solid electrolytic capacitor according to thepresent invention is excellent in heat resistance.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the scopeof the present invention being interpreted by the terms of the appendedclaims.

1. A method of manufacturing a solid electrolytic capacitor, comprisingthe step of winding an anodic foil provided with a dielectric film and acounter cathodic foil through a separator, and further comprising thestep of dipping said separator in a solution containing a silanecoupling agent in advance of the step of winding said anodic foil andsaid counter cathodic foil through said separator.
 2. The method ofmanufacturing a solid electrolytic capacitor according to claim 1,employing a supercritical fluid as a solvent in the step of dipping saidseparator in said solution containing a silane coupling agent.
 3. Themethod of manufacturing a solid electrolytic capacitor according toclaim 1, further comprising the step of performing heat treatment onsaid separator in a vacuum after the step of dipping said separator insaid solution containing a silane coupling agent.
 4. The method ofmanufacturing a solid electrolytic capacitor according to claim 3,performing said heat treatment in said vacuum in the temperature rangeof 80 to 200° C.